1. Field of the Invention
This invention relates generally to envelope detectors and, more particularly, to a high-gain, high-speed full wave bridge envelope detector suitable for wide band input signal applications and which can be fabricated in monolithic integrated circuit form.
2. Description of the Prior Art
Full wave rectifiers are well known and are used in a variety of applications. For example, full wave rectifiers are commonly used in amplitude demodulation, amplitude measurement systems, and may be utilized for video detection and included as part of the video detector portion of a television receiver to extract the video signal from the television composite carrier signal.
Certain known rectifiers capable of being used in high speed, wide band input signal applications generally require direct current biasing at a quiescent operating point. Unfortunately, this may cause the full wave rectifier to exhibit a non-linear transfer characteristic. Another problem associated with certain prior art devices which were utilized, for example, as video detectors in a television receiver, resides in the fact that a tuned circuit at the television carrier frequency is required. This tuned circuit may cause phase related problems in a television receiver as well as spurious feedback to the high sensitivity input terminals. Furthermore, the tuned circuit requires additional external terminals.
The circuit described in U.S. patent application Ser. No. 220,329 filed Dec. 29, 1980, now U.S. Pat. No. 4,336,586, entitled "Linear Full Wave Rectifier Circuit" and assigned to the assignee of the present invention represents a solution to the above identified problems; i.e. the circuit produces a nearly perfect full wave rectified version of an applied alternating signal. The circuit comprises first and second identical non-linear symmetrical rectifiers each having a respective output coupled to a current mirror circuit. In response to the alternating signal being applied to the input of the first rectifier, a full wave rectified signal is produced at the output thereof for driving the current mirror. The output of the current mirror, which is coupled to the output of a second rectifier, represents a signal identical to the output from the first rectifier. This signal is utilized to produce an input signal to the second rectifer to render the output signal therefrom identical to the output signal from the current mirror circuit. Thus, the input signal to the second rectifier is a perfect full wave rectified version of the input signal applied to the first rectifier.
Unfortunately, a problem exists in generating enough drive at the output of the television system IF amplifier in order to obtain a sufficient output from the rectifier circuit.